The present invention relates to an electroacoustic transducer such as a loud speaker and receiver used in a portable communication equipment.
In recent years, a small dynamic speaker having a thin thickness and a good performance is widely used as an electroacoustic transducer in a portable communication equipment such as a portable telephone.
A conventional speaker will be described hereinafter with reference to drawings.
FIG. 7 is a plan view showing a conventional dynamic speaker, FIG. 8 is a sectional view taken along a line VIII—VIII of FIG. 7, and FIG. 9 is a plan view as viewed from the underside of the speaker.
The dynamic speaker 1 has an elliptic frame 2 made of resin. The frame 2 comprises a bottom 3 having a circular hole 4 and a peripheral projection 5 having an annular step 6. An annular yoke 7 having a central projection 8 is secured to the bottom 3 of the frame 2. An annular permanent magnet 10 is secured to the yoke 7, and an annular top plate 11 is mounted on the permanent magnet 10. Outside walls of the yoke 7, permanent magnet 10 and top plate are contacted with the inside wall 5a of the peripheral projection 5 of the frame 2.
An elliptic diaphragm 13 made of plastic has a central spherical portion 13a, annular peripheral portion 13b and an annular peripheral flat portion 13c. The peripheral flat portion 13c is secured to the step 6 of the peripheral projection 5 of the frame 2.
An annular voice coil 15 is secured to the underside of the diaphragm 13. Both ends of the voice coil 15 are connected to terminal plates 16 provided on the underside of a pair of projections 17 of the frame 2. The voice coil 15 is inserted in a magnetic gap 18 formed between the central projection 8 of the yoke 7 and the top plate 11. A protector 20 made of a light metal plate is secured to the peripheral flat portion 13c of the diaphragm 13 at a peripheral edge 20a by an adhesive 21. A plurality of sound discharge holes 22 are formed in the protector 20, communicating with an upper space 23, and a pair of sound discharge holes 24 are formed in the bottom 3 of the frame 2, communicating with a lower space 25.
In the above described conventional speakers, the diaphragm 13 is secured to the step 6 of the peripheral projection 5. Therefore, the diameter of the diaphragm is reduced by the width of the step 6. Therefore, a large magnetic force can not be obtained, and hence a sufficient sound pressure can not be produced.